Subject: printk: add console handover
From: John Ogness <john.ogness@linutronix.de>
Date: Mon Nov 30 01:42:09 2020 +0106

From: John Ogness <john.ogness@linutronix.de>

If earlyprintk is used, a boot console will print directly to the
console immediately. The boot console will unregister itself as soon
as a non-boot console registers. However, the non-boot console does
not begin printing until its kthread has started. Since this happens
much later, there is a long pause in the console output. If the
ringbuffer is small, messages could even be dropped during the
pause.

Add a new CON_HANDOVER console flag to be used internally by printk
in order to track which non-boot console took over from a boot
console. If handover consoles have implemented write_atomic(), they
are allowed to print directly to the console until their kthread can
take over.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>



---
 include/linux/console.h |    1 +
 kernel/printk/printk.c  |   15 +++++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)
---
@ include/linux/console.h:146 @ static inline int con_debug_leave(void)
 #define CON_ANYTIME	(16) /* Safe to call when cpu is offline */
 #define CON_BRL		(32) /* Used for a braille device */
 #define CON_EXTENDED	(64) /* Use the extended output format a la /dev/kmsg */
+#define CON_HANDOVER	(128) /* Device was previously a boot console. */
 
 struct console {
 	char	name[16];
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@ include/linux/console.h:1749 @ static bool console_may_sync(struct cons
 		return false;
 	if (con->write_atomic && kernel_sync_mode())
 		return true;
+	if (con->write_atomic && (con->flags & CON_HANDOVER) && !con->thread)
+		return true;
 	if (con->write && (con->flags & CON_BOOT) && !con->thread)
 		return true;
 	return false;
@ include/linux/console.h:1766 @ static bool call_sync_console_driver(str
 		return true;
 	}
 
-	if (con->write && (con->flags & CON_BOOT) && !con->thread) {
+	if (con->write_atomic && (con->flags & CON_HANDOVER) && !con->thread) {
+		if (console_trylock()) {
+			con->write_atomic(con, text, text_len);
+			console_unlock();
+			return true;
+		}
+
+	} else if (con->write && (con->flags & CON_BOOT) && !con->thread) {
 		if (console_trylock()) {
 			con->write(con, text, text_len);
 			console_unlock();
@ include/linux/console.h:2903 @ void register_console(struct console *ne
 	 * the real console are the same physical device, it's annoying to
 	 * see the beginning boot messages twice
 	 */
-	if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV))
+	if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV)) {
 		newcon->flags &= ~CON_PRINTBUFFER;
+		newcon->flags |= CON_HANDOVER;
+	}
 
 	/*
 	 *	Put this console in the list - keep the